To read/write on the database (whichever you're using), with NodeBB I'd suggest you to use the "abstract layer" they developed. You can import src/database.js from the sources and use the native functions implemented in nodebb.

To see how they structure data, I'd suggest you to go check the code of their official plugins (or directly of the platform core).

Unless you are doing something specific never write directly to the document store (its not as db as you know it) for one thing is not the best-structured store owing to the fact they support multiple.

As @Giggiux says use the abstraction layer this allows your plugin to work with both mongo and redis and it also allows NodeBB to change the structure of the doc store or even add more and your plugin will just work.

You could replicate and modify what in NodeBB are categories as groups (Movie/games/tv show), and what in NodeBB are topics as actual elements (a movie, a tv-series, a game). The best examples of code are their one and you can find them in /src

In order to use these things in your plugin, check official plugins code ^^